The teenage gunman who killed a classmate at an Oregon high school on Tuesday was armed with a military-style rifle and a semiautomatic pistol obtained from his home and lacked any known connection to his victim, police said on Wednesday.

A 17-year-old boy's sinister plot to murder his family and then attack his school with pressure-cooker bombs, Molotov cocktails and deadly firearms to kill as many students as he could has been uncovered in the US.

A school district spokesman says a shooting on a street in front of an elementary school in Ohio led to a precautionary lockdown of the building, but no children were injured or involved in the shooting.